Joining of dissimilar biocompatible metals is attractive for a variety of reasons such as the introduction of novel functionalities through the incorporation of shape memory or superelastic materials or for the selective use of exceptionally inert materials such as Pt for critical and long-term implant applications. One reason there is interest in joining dissimilar biocompatible metals for medical devices is due to unique properties and functionalities possessed by materials such as NiTi (shape memory and superelastic properties), Pt (inertness, electrical conductivity), and stainless steel (biocompatibility, low cost). The need to join these materials stems from the desire to integrate their unique properties in a robust and cost-effective manner.
Traditional joining processes have limited spatial selectivity and large heat inputs (arc welding, etc) which promote brittle phase formation. Brazing using a filler material with a lower melting temperature than either of the base materials can eliminate melting of the base metals and can potentially avoid intermetallic formation but requires careful selection of the filler material. This may be particularly difficult in medical devices due to the required biocompatibility. For example, laser brazing of NiTi shape memory alloys to stainless steel using silver-based filler materials can cause corrosion resistance of the joint to be worse than that of the base materials.
Solid-state processes can also be performed which may allow for greater control over material mixing. One such process, diffusion bonding for direct bonding of biocompatible material pairs such as NiTi/SS, results in the formation of brittle intermetallics. Diffusion bonding of the same material pair using Ni and Cu interlayers also observed the formation of intermetallics at the joint interfaces. In addition, this bonding process can require heating of the entire device to elevated temperatures and, like ultrasonic welding, the ability to impart a compressive stress on the joint. These requirements can make such process difficult for medical devices with heat-sensitive components, small size, and complex joint geometries.
Traditional joining methods including mechanical joints, such as riveting and crimping may have problems with sterilization, hermetic sealing, and crevice corrosion, for instance. Diffusion bonding may require significant processing time, unnecessary heating of potentially heat-sensitive components, and like ultrasonic welding, may be impossible to perform on some joint geometries due to the need for mechanical loading of the joint. Traditional brazing joints require the addition of filler materials which can increase process complexity and cost with the added issue of potentially non-biocompatible filler materials. Adhesives have been used for use in medical devices but may not be acceptable for long-term implantation applications. Fusion welding processes can result in excessive heat input and mixing of the base materials which can promote brittle intermetallic phase formation in dissimilar metal pairs.